Add channel support to api.dartlang.org

This CL adds the following URL schemes:
  - http://api.dartlang.org/docs/channels/{be,dev,stable}/latest/
  - http://api.dartlang.org/docs/channels/{be,dev,stable}/424242/

Our channel builders are archiving the dartdocs to the following location:
gs://dartlang-api-docs/channels/{be,dev,stable}/REVISION/

Note:

This is a non-intrusive change, so the current URL schemes stay and the behavior is the same as before.

I've had problems testing this locally on my machine:  dev_appserver.py has issues with storage modules (e.g. files).

If everything works out as expected, I'll do another CL to map the existing URL schemes to the dartdoc archived by our channel builders.